Submarine power feeding system having submarine feeding cables and power feeding branching devices

ABSTRACT

A submarine power feeding branching device comprises a constant current-constant current converter which isolates an input side for a trunk submarine cable from an output side for a branch submarine cable. The converter receives a first constant current and produces a second constant current by using the first constant current. The second constant current is supplied to the output side while the first constant current is returned to the input side. Because the input side and the output side are isolated, it is easy to add/remove the device to/from a submarine power feeding system. Intensity of the second constant current can be controlled by controlling duty ratios of switches included in the converter. Thus, it is possible that the intensity of the second constant current is equal to that of the first constant current. Therefore, a submarine repeater can be provided along either the trunk cable or the branch cable.

CROSS-REFERENCE TO RELATED PATENT APPLICATIONS

This application is a division of application Ser. No. 10/687,931, filedOct. 20, 2003, now U.S. Pat. No. 7,166,933, which claims priority fromJapanese Patent Application No. 2002-305918, filed Oct. 21, 2002, by JunMuramatsu, Kenichi Asakawa, Katsuyoshi Kawaguchi, both of which areincorporated herein by reference in their entirety. This applicationclaims only subject matter disclosed in the parent application andtherefore presents no new matter.

BACKGROUND OF THE INVENTION

This invention relates to a submarine electric power feeding branchingdevice and a submarine electric power feeding system using the submarineelectric power feeding branching device. In particular, this inventionrelates to a submarine power feeding branching device suitable forarranging submarine cables in mesh pattern and constructing a submarinepower feeding system having the submarine cables arranged in meshpattern.

In fields of researches for submarine earthquakes, ocean environment orthe like, there are demands for arranging a large number of submarineobservation devices, such as seismometers, tsunami instruments, currentmeters, hydrometers or the like, in two dimensional arrangement (or amatrix) on the bottom of the sea to collect various data from thesubmarine observation devices.

To meet such demands, it is possible to construct a observation systemthat comprises submarine observation devices, which are provided on thebottom of the sea, and submarine cables, which are used for feedingelectric power to the submarine observation devices and continuouslycollecting data from (or communicating with) the submarine observationdevices.

However, it is impractical that the submarine cables individuallyconnect the submarine observation devices to a land observationdevice(s). Furthermore, when an observation system has plural submarineobservation devices which are connected to a submarine cable in series,it possesses low reliability. This is because the submarine observationdevices located between a failure point and the end of the submarinecable can not receive electric power from a land observation device andcommunicate with the land observation device when the failure occurs inthe submarine cable. Thus, a submarine cable system (or power feedingsystem) having submarine cables arranged in mesh or lattice pattern isnecessary to construct an observation system having a large number ofsubmarine observation devices arranged in two dimensional arrangement(or a matrix) and possessing high reliability.

SUMMARY OF THE INVENTION

It is therefore an object of this invention to provide a submarineelectric power feeding branching device capable of constructing asubmarine electric power feeding system having submarine cables arrangedin mesh pattern and possessing high reliability.

Other objects of this invention will become clear as the descriptionproceeds.

According to a first aspect of this invention, a power feeding branchingdevice comprises a constant current-constant current converter having aninput terminal, a first output terminal and a second output terminalwhich is electrically isolated from both the input terminal and thefirst output terminal. A controller is connected to the constantcurrent-constant current converter and makes the constantcurrent-constant current converter utilize a first constant currentsupplied to the input terminal to produce a second constant current anda restored first constant current. The second constant current and therestored first constant current are to be supplied to the second outputterminal and the first output terminal, respectively.

The power feeding branching device may further comprise a bypass circuitconnected between the input terminal and the first output terminal. Thebypass circuit bypasses the constant current-constant current converterto allow the first constant current instead of the restored firstconstant current to lead from the input terminal to the first inputterminal when the input terminal has an electrical potential higher thana predetermined potential.

Furthermore, the power feeding branching device may comprises a bypassdiode connected between the second output terminal and a groundterminal.

According to a second aspect of this invention, a power feeding systemincludes a plurality of trunk cables connected to feeding devices, aplurality of branch cables each of which is provided between adjacenttwo of the trunk cables, and a plurality of power feeding branchingdevices for connecting the branch cables with the trunk cables. Each ofthe power feeding branching devices comprises a constantcurrent-constant current converter having an input terminal, a firstoutput terminal and a second output terminal which is electricallyisolated from both the input terminal and the first output terminal. Acontroller is connected to the constant current-constant currentconverter and makes the constant current-constant current converterutilize a first constant current supplied to the input terminal toproduce a second constant current and a restored first constant current.The second constant current and the restored first constant current areto be supplied to the second output terminal and the first outputterminal, respectively.

In the power feeding system, the power feeding branching devices areclassified into two types. One of the types leads the second constantcurrent from the constant current-constant current converter to thesecond output terminal. The other of the types leads the second constantcurrent from the second output terminal to constant current-constantcurrent converter. Each of the branch cables is connected between twosecond output terminals of two of the power feeding branching devicesdifferent from each other in type.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a communication submarine cable system;

FIG. 2 is a block diagram of a conventional submarine cable powerfeeding system;

FIG. 3 is a circuit diagram of a current limiter used in a submarinebranching device for the submarine cable power feeding system of FIG. 2;

FIG. 4 is a block diagram of a related submarine electric power feedingsystem;

FIG. 5 is a block diagram of a submarine electric power feeding systemaccording to a first embodiment of this invention;

FIG. 6 is a schematic circuit diagram of a submarine power feedingbranching device used in the submarine electric power feeding system ofFIG. 5;

FIG. 7 is a waveform chart for describing an operation of the submarinepower feeding branching device of FIG. 6 in a case where switchesincluded in the submarine power feeding branching device have a dutyratio of 50%;

FIG. 8 is another waveform chart for describing the operation of thesubmarine power feeding branching device of FIG. 6 in a case where theswitches have another duty ratio under 50%;

FIG. 9 is still another waveform chart for describing the operation ofthe submarine power feeding branching device of FIG. 6 in a case wherethe switches have different duty ratios;

FIG. 10 is a graph representing a relation between a duty ratio Rd1 anda ratio of an output current to an input current;

FIG. 11 is a graph representing measured output voltage-currentcharacteristics of a current-current converter as used in the submarinepower feeding branching device of FIG. 6;

FIG. 12 is a circuit diagram of a measurement circuit for measuring theoutput voltage-current characteristics shown in FIG. 11;

FIG. 13 is a graph for describing an operation point of a combineddevice of two submarine power feeding branching devices as shown in FIG.6.

FIG. 14 is a schematic circuit diagram of a submarine power feedingbranching device according to a second embodiment of this invention;

FIG. 15 is a schematic circuit diagram of a submarine power feedingbranching device according to a third embodiment of this invention; and

FIG. 16 is a schematic circuit diagram of a submarine power feedingbranching device according to a fourth embodiment of this invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Description will be at first directed to a conventional submarine cablesystem. As the conventional submarine cable system, a communicationsubmarine cable system is well known.

The communication submarine cable system comprise a long communicationsubmarine cable including a power feeder inside to feed electric powerfor repeaters provided along the communication submarine cable. Thepower feeder comprises a single conductor to reduce diameter and cost ofthe communication submarine cable. Seawater is used as a return circuitin the communication submarine cable system. The communication submarinecable system adopts a constant current feeding system to make easyinsulation between circuits in each repeater and strengthen tolerancefor failure of a short circuit of the submarine cable.

If the repeaters are fed with constant voltage, an electronic circuit ineach repeater must have a grounded terminal. Accordingly, the electroniccircuit has a higher voltage part and a lower voltage part. Therefore,the electronic circuit needs expensive electronic components which canwithstand high voltage. Furthermore, dimensions of the electroniccircuit tend to become large to maintain reliability regardinginsulation between the electronic circuit and the seawater.

As it is, the electronic circuit in each repeater need not have theground terminal because the repeaters are fed with constant current.Accordingly, differences of voltages are relatively small in theelectronic circuit and the expensive electronic components which canwithstand the high voltage are unnecessary for the electronic circuit.Furthermore, the electronic circuit is easily insulated from theseawater by covering it in whole with an insulator. In addition, theconstant current feeding system can feed the constant current as far asa short circuit failure point from land feeding device through thesubmarine cable which is short circuited. In a case of the constantvoltage feeding system, the communication submarine cable system isconsiderably affected by drop of electrical potential at the shortcircuit failure point.

Referring to FIG. 1, the description is made about an existingcommunication submarine cable system having submarine branching devices.

In FIG. 1, the communication submarine cable system comprises submarinebranching devices 11 a and 11 b, submarine cables 12 a-12 e and feedingdevices 13 a-13 d. While the submarine cables 12 a, 12 e and 12 d serveas a main submarine cable, each of the submarine cables 12 b and 12 cserves as a branch submarine cable.

It may seem that the communication submarine cable system is applicableto a hydrographic observation system. However, such a system cancomprise a comb submarine cable network but it can not comprise asubmarine cable network having a lattice or mesh arrangement. This isbecause each branch cable 12 b, 12 c must be connected to the feedingdevice 13 b, 13 c at one end and grounded to the seawater at the otherend.

FIG. 2 is a block diagram of a conventional submarine cable powerfeeding system designed with referring to the above mentionedcommunication submarine cable system.

The system of FIG. 2 comprises a land observation device 21 located onland, submarine branching devices 22 a, 22 b located on the bottom ofthe sea, and submarine observation devices 23 a-23 e also located on thebottom of the sea. While the land observation device 21 comprises a landfeeding device 24, each of the submarine branching devices 22 a, 22 bcomprises a current limiter 25 a, 25 b.

The land feeding device 24 feeds a first constant current to a trunkfeeder line 26. Upon receiving the first constant current, eachsubmarine branching device 22 a, 22 b uses the current limiter 24 a, 24b to feed a second constant current to a branch feeder line 27 a, 27 b(or the submarine observation device 23 a-23 b, 23 c-23 d).

The current limiter 25 a is composed, for example, as illustrated inFIG. 3. The current limiter 25 b is similar to the current limiter 25 ain structure. In FIG. 3, the feeding current on the trunk feeder line 26is equal to 1 [A] while the feeding current on the branch feeder line 27a is equal to 0.1 [A]. However, the feeding current on the branch feederline 27 a is varied by adjusting a variable resistor RV. That is, thefeeding current on the branch feeder line 27 a is decided by the Zenervoltage of a Zener diode RC1 and a resistance of the variable resistorRV. An emitter current of a transistor TR is held constant because theZener voltage is stabilized. Such a submarine power feeding system isdisclosed in Japanese Unexamined Patent Publication No. 2001-309553.

The submarine power feeding system has some problems as follow.

First, as understood from FIG. 2, the submarine power feeding system hasa comb shape. That is, use of the submarine branching device as shown inFIG. 3 makes possible to contract a comb shaped submarine power feedingsystem. However, it is hard to construct a lattice or mesh arrangementof cables for a submarine power feeding system by the use of thesubmarine branching devices. This is because the submarine branchingdevices that are provided to adjacent branch feeding lines must formpairs to construct the lattice arrangement of the cables. In such acase, it is difficult to match a first current produced by one of eachpair with a second current produced by the other of the pair though thefirst current must be matched with the second current.

Secondly, the submarine power feeding system of FIG. 2 has disadvantageof difficulty in adding and removing the submarine branching device(s).This is because each branching device (or the variable resistor thereof)must be adjusted in response to variation of the load though it islocated on the bottom of the sea.

Thirdly, the submarine power feeding system can not operate when thetrunk feeder line 26 is short circuited. This is because the trunkfeeder line 26 has an electric potential of about 0 [V] and thesubmarine branching devices 22 a and 22 b are inoperative when the trunkfeeder line 26 is short circuited.

Fourthly, the submarine branching device is an inefficient device. Thisis because the submarine branching device uses resistors to limits thefeeding current on the branch feeder line. That is, the resistors wasteelectric power. Additionally, the submarine branching device must bedesigned with consideration of heat radiated by the resistors.

Fifthly, the submarine power feeding system needs plural submarinebranching devices which have different specifications. This is becausethe second constant current on the branch feeder line is different fromthe first constant current on the trunk feeder line.

Sixthly, the submarine branching device is not insulated between aninput side and an output side. Electrical potential difference betweenthe input side and the output side of the submarine branching devicemust be smaller than withstand voltage of electronic devices of thesubmarine branching device. In other words, the submarine branchingdevice restricts freedom of design of the submarine power feedingsystem.

A proposal has been made about a submarine electric power feeding systemhaving submarine cables arranged in mesh pattern. Such a submarine powerfeeding system is disclosed in Japanese Unexamined Patent PublicationNo. 2003-244032.

FIG. 4 shows the proposed submarine electric power feeding system. Thesubmarine power feeding system comprises main backbone cables 41 a and41 b connected to land constant voltage feeding devices 42 a and 42 brespectively. Sub backbone cables 43 a, 43 b and 43 c are connected tothe main backbone cables 41 a and 41 b by the use of submarine powerfeeding branching devices 44 a and 44 b. Submarine repeaters 45 areconnected to any one of the sub backbone cables 43 a, 43 b and 43 cbetween any one pair of the submarine power feeding branching devices 44a and 44 b. A submarine observation device 46 is connected to each ofthe submarine repeaters 45.

The submarine power feeding system of FIG. 4 adopts a constant voltagefeeding system for the main backbone cables and a constant currentfeeding system for the sub backbone cables.

Because of the constant voltage feeding system, each of the submarinepower feeding branching devices 44 a and 44 b must be grounded. Thismeans that each of the submarine power feeding branching devices has acircuit including higher voltage circuitry and lower voltage circuitcircuitry. Accordingly, expensive electronic components which canwithstand high voltage are necessary for the power feeding branchingdevices. Furthermore, the circuit of the power feeding branching devicetends to be large to maintain insulation between itself and seawater.

Referring to FIGS. 5 to 11, description will proceed to a submarineelectric power feeding system according to a first embodiment of thisinvention.

FIG. 5 shows a block diagram of the submarine electric power feedingsystem. The submarine electric power feeding system comprises aplurality of constant current feeding device 51 a, 51 b and 53 c whichare provided in land stations (not shown) located apart from oneanother.

The constant current feeding device 51 a, 51 b and 51 c is connected totrunk submarine cables 52 a, 52 b and 52 c respectively. The trunksubmarine cables 52 a, 52 b and 52 c generally extends offshore. Thegreater part of each of the trunk submarine cables 52 a, 52 b and 52 cis placed on the bottom of the sea.

Two types of submarine power feeding branching devices 53 a and 53 b areprovided along each of the trunk submarine cables 52 a, 52 b and 52 c.In other words, the submarine power feeding branching devices 53 a and53 b are interposed in each of the trunk submarine cables 52 a, 52 b and52 c. The submarine power feeding branching device 53 a having a firsttype and the submarine power feeding branching device 53 b having asecond type are fundamentally similar to each other in structure.However, the first type 53 a produces constant current flowing from theinside to an output terminal thereof while the second type 53 b producesconstant current flowing from an output terminal thereof to the inside.

Each of the submarine power feeding branching devices 53 a is acounterpart of any one of the submarine power feeding branching devices53 b. In other words, the first type 53 a and the second type 53 b ofthe submarine power feeding branching devices make a pair. Companions ofeach pair of the submarine power feeding branching devices 53 a and 53 bare interposed in adjacent two of the trunk submarine cables 52 a-52 c.For instance, the submarine power feeding branching device 53 ainterposed in the trunk submarine cable 52 a is a companion to thesubmarine power feeding branching device 53 b interposed in the trunksubmarine cable 52 b. Besides, the submarine power feeding branchingdevice 53 a interposed in the trunk submarine cable 52 b is a companionto the submarine power feeding branching device 53 b interposed in thetrunk submarine cable 52 c.

Branch submarine cables 54 a, 54 b, 54 c and 54 d are connected betweencompanions of the pairs of the submarine power feeding branching devices53 a and 53 b. The branch submarine cables are generally placed on thebottom of the sea. The branch submarine cables may be approximatelyperpendicular to the trunk submarine cables 52 a, 52 b and 52 c. Thetrunk submarine cables and the branch submarine cables are ideallyarranged in mesh or lattice pattern. However, the configuration of thetrunk and the branch submarine cables is not limited in the meshpattern. The configuration is changed because of not only landform ofthe seabed but also the other factors. The branch submarine cables andthe trunk submarine cables serve a net or mesh power feeding line.

Submarine repeaters 55 are interposed in (or provided along) the brunchsubmarine cables 54 a, 54 b, 54 c and 54 d. The repeaters 55 areconnected to submarine observation devices 56. The submarine observationdevices 56 are placed on the bottom of the sea. The submarineobservation devices 56 may be arranged in second dimensional arrangement(or a matrix).

With this structure, the land constant current feeding devices 51 a, 51b and 51 c feed first constant currents to the submarine power feedingbranching devices 53 a and 53 b through the trunk submarine cables 52 a,52 b and 52 c.

Each of the submarine power feeding branching devices 53 a feeds asecond constant current to the branch submarine cable 54 a, 54 b, 54 cor 54 d when it receives the first constant current fed from theconstant current feeding device 51 a, 51 b or 51 c. On the other hand,each of the submarine power feeding branching devices 53 b absorbs thesecond constant current fed from the submarine power feeding branchingdevices 53 a which companions thereto. Here, the seawater is used as areturn circuit for each of the trunk and the branch submarine cables.

The submarine repeaters 55 have a structure well known in the art. Eachof the submarine repeaters 55 produces constant voltage from the secondconstant current fed from the submarine power feeding branching devices53 a to feed it for the submarine observation device 56 connectedthereto.

The submarine observation devices 56 also have a structure well known inthe art. While each of the submarine observation devices 56 receives theconstant voltage fed from the submarine repeater 55 connected thereto,it performs regular observation and produces observation data. Theobservation data produced by the submarine observation devices 56 aretransmitted to a land observation device(s) provided in the landstation(s) through the branch submarine cable(s) and trunk submarinecable(s).

Because the submarine power feeding system adopts the constant currentfeeding system for the trunk submarine cables 52 a, 52 b and 52 c, aplurality of the submarine power feeding branching devices 53 a and 53 bcan be connected in series. Therefore, it is easy to extend the trunksubmarine cables 52 a, 52 b and 52 c and add additional submarine powerfeeding branching devices. In addition, it is easy to provide additionalsubmarine repeaters along the trunk submarine cables 52 a, 52 b and 52 ctogether with additional observation devices. Thus, the submarine powerfeeding system can be expanded over a wide area with a mesh pattern ofsubmarine cables as illustrated in FIG. 5.

The submarine power feeding system also uses the constant currentfeeding system for feeding electric power to the branch submarine cables54 a, 54 b, 54 c and 54 d. Therefore, it is easy to provide additionalsubmarine repeaters along the branch submarine cables 52 a, 52 b and 52c together with additional observation devices.

Each of the submarine repeaters is fed with electrical power from two ofthe submarine feeding branching device 53 a and 53 b. Accordingly, thesubmarine power feeding system has tolerance for failure of a shortcircuit. Theoretically, even when only one short circuit occurs on thetrunk and the branch submarine cables of the submarine power feedingsystem, all of the submarine repeaters provided to the branch submarinecables 54 a-54 d can be fed with electric power.

Next, the submarine power feeding branching device 53 a will bedescribed in more detail with referring to FIG. 6.

FIG. 6 shows an internal construction of the submarine power feedingbranching device 53 a. As illustrated in FIG. 6, the submarine powerfeeding branching device 53 a comprises a constant current-constantcurrent converter 61, a switch controller 62, a communication device 63and a bypass circuit 64. The submarine power feeding branching device 53a is housed in a pressure-resistant case (not shown).

The constant current-constant current converter 61 has a pair of input(or primary) side terminals and a pair of output (or secondary) sideterminals. The input side terminals are electrically isolated from theoutput side terminals. The input side terminals of the constantcurrent-constant current converter 61 are connected to an input terminal65 and a first output terminal 66 of the submarine power feedingbranching device 53 a. The output side terminals of the constantcurrent-constant current converter 61 serve as a second output terminal67 and the ground terminal 68 of the submarine power feeding branchingdevice 53 a.

The constant current-constant current converter 61 comprises atransformer TR1 having primary and secondary windings. The primarywinding is connected to the input terminal 65 at the midpoint thereofwhile the second winding is connected to the ground terminal 68 at themidpoint thereof. A first condenser C1 is connected between the inputterminal 65 and the first output terminal 66. A first switch S1 isconnected between one end of the primary winding and the first outputterminal 66. A second switch S2 is connected between the other end ofthe primary winding and the first output terminal 66. Semiconductorswitches, such as MOSFETs, bipolar transistors or the like, may be usedfor the first and the second switches S1 and S2. A first diode D1 isconnected between one end of the secondary winding and the second outputterminal 67 while a second diode D2 is connected to the other end of thesecondary winding and the second output terminal 67. A second condenserC2 is connected between the second output terminal 67 and the groundterminal 68. A bypass diode 611 is connected between the second outputterminal 67 and the ground terminal 68.

While the first condenser C1 and the first and the second switches S1and S2 form a square waveform producing portion. The first and thesecond diode D1 and D2 and the second condenser C2 forms a rectifyingsmoothing portion.

The switch controller 62 controls each of the switches S1 and S2 to makeit an on state or an off state. When the first switch S1 is in the onstate and the second switch S2 is in the off state, the current suppliedto the input terminal 65 flows in the primary winding of the transformerTR1 as shown by a solid line arrow N1-1. At this time, a secondary sidecurrent flows in the secondary winding of the transformer TR1 as shownby a solid line arrow N2-1. On the other hand, when the first switch S1is in the off state and the second switch S2 is in the on state, thecurrent supplied to the input terminal 65 oppositely flows in theprimary winding of the transformer TR1 as shown by a broken line arrowN1-2. At this time, the secondary side current flows in the secondarywinding of the transformer TR1 as shown by a broken line arrow N2-2.Thus, the switch controller 62 produces square wave currents in theprimary winding of the transformer TR1 by controlling the switches S1and S2. The square wave currents are added to each other and returns tothe first constant current. In other words, the square waveformproducing portion produces a restored first constant current from thesquare wave currents to supply it to the first output terminal 66.

The bypass circuit 64 comprises a switching circuit 641 and a voltagedetecting circuit 642 for controlling the switch circuit 641. Theswitching circuit 641 is normally in an off state. The voltage detectingcircuit 642 detects a voltage difference between the input terminal 65and the first output terminal 66. The voltage detecting circuit 642makes the switching circuit 641 an on state when it detects the voltagedifference equal to or larger than a predetermined value.

The communication device 63 is connected to the land observation deviceor the like provided in the land station (not shown) through, forexample, optical fibers provided in the submarine trunk cables and thesubmarine branch cables. In addition, the communication device 63 isconnected to the switch controller 62 and the voltage detecting circuit642. The communication device 63 receives a control signal (or acommand) transmitted from the land observation device to send it for theswitch controller 62 and/or the voltage detecting circuit 642.Furthermore, the communication device 63 transmits a measurement signalrepresenting measured results concerning voltage and current at anypoints in the submarine power feeding branching device 53 a.

The description will be soon made about the operation of the submarinepower feeding branching device 53 a. Hereinafter, it is assumed that thesubmarine power feeding branching device 53 a is connected to the trunksubmarine cable 52 a and the branching submarine cable 54 a.

The first constant current fed to the input terminal 65 through thetrunk submarine cable 52 a is supplied to both of the condenser C1 andthe midpoint of the primary winding of the transformer TR1. While theswitches S1 and S2 are alternately repeatedly changed between the on andthe off states, primary current with the square waves flows in theprimary winding of the transformer TR1.

The switch controller 62 controls the switches S1 and S2 in a manner asdescribed later in more detail to generate the square waves of theprimary side current in the primary winding of the transformer TR1.

The condenser C1 absorbs the first constant current flowing in the trunksubmarine cable 52 a to prevent abnormal high voltage from occurring atthe input terminal 65 in a case where both of the switches S1 and S2 arein the off state. The condenser C1 further prevents noises produced bythe operation of the switches S1 and S2 from being transmitted to thetrunk submarine cable 52 a.

The transformer TR1 isolates between the trunk submarine cable 52 a andthe branch submarine cable 54 a while it supplies power of the primary(or input) side thereof to the secondary (or output) side thereof. Thatis, the transformer TR1 produces the secondary side current with squarewaves corresponding to the square waves of the primary side current.

A combination of the diodes D1 and D2 and the capacitor C2 rectifies andsmoothes the secondary side current and produces an output constantcurrent. The output constant current is supplied to the branch submarinecable 54 a through the second output terminal 67 as the second constantcurrent.

The bypass diode 611 bypasses a surplus current fed from the submarinepower feeding branching device 53 b which companions to the presentsubmarine power feeding branching device 53 a through the branchsubmarine cable 54 a, when the output current is smaller than that ofthe submarine power feeding branching device 53 b. The surplus currentmay occur when the submarine power feeding branching device 53 a has afailure(s) anywhere. The bypass diode 611 has a cathode connected to thesecond output terminal 67. In the submarine power feeding branchingdevice 53 b, a bypass diode comprises an anode connected to a secondoutput terminal (i.e. the branch submarine cable 54 a) differently fromthe bypass diode 611.

The bypass circuit 64 detects overvoltage of the input terminal 101 tobypass the first constant current supplied to the input terminal 65 forthe first output terminal 66. The overvoltage may occur when thesecondary wiring is opened or when each switch S1 or S2 and/or theswitching controller 62 is out of order. In such a case, the firstconstant current supplied to the input terminal 65 bypasses the constantcurrent-constant current converter 61 and thereby the bypass circuit 64prevents excessive voltage from being given to the submarine powerfeeding branching device 53 a.

The voltage detecting circuit 642 detects the overvoltage of the inputterminal 101 to control the switching circuit 641. That is, the voltagedetecting circuit 642 supplies a control signal for the switchingcircuit 641 when it detects the overvoltage of the input terminal 101.The switching circuit 641 is normally in the off state as mentionedabove. Upon receiving the control signal from the voltage detectingcircuit 642, the switching circuit 641 changes from the off state to theon state. Thus, the constant current-constant current converter isbypassed by the bypass circuit 64.

After the overvoltage is detected once, the voltage detecting circuit642 keeps the switching circuit 641 being in the on state until itreceives a command signal transmitted from the land station through thecommunication device 63. This is made to prevent the switching circuit641 from chattering. When the voltage detecting circuit 642 receives thecommand signal from the land station, it returns the switching circuitto the original (i.e. off or open) state.

Next, the operation of the switches S1 and S2 will be described withreferring to FIGS. 7 and 8.

FIG. 7 is a timing chart in a case where the switch S1, S2 has a dutyratio of 50% each. The duty ratio Rd is defined by:Rd=(Ton/T)×100 [%]  (1).

In this case, the secondary current has the square waves with intensityIout when the transformer TR1 has a turns ratio of N1/N2. The intensityIout is given by:Iout=N1/N2×Iin  (2).

FIG. 8 is a timing chart in a case where the switch S1 and S2 has acommon duty ration smaller than 50%. As shown in the bottom of FIG. 8,the input terminal 65 receives the first constant current with theintensity Iin regardless of the switches S1 and S2. Accordingly, thecondenser C1 is charged with the first constant current Iin when boththe switches S1 and S2 are in the off state. If either the switch S1 orS2 turns into the on state, electric charges charged in the condenser C1is supplied to the transformer TR1. At this time, the primary sidecurrent flowing in the primary winding of the transformer TR1 hasintensity I1 given by:I1=T/(2Ton)×Iin  (3).

Furthermore, the secondary side current flowing in the secondary windingof the transformer TR2 has intensity I2 given by:

$\begin{matrix}\begin{matrix}{{I\; 2} = {N\;{1/N}\; 2 \times I\; 1}} \\{= {N\;{1/N}\; 2 \times {T/\left( {2\;{Ton}} \right)} \times I\; i\;{n.}}}\end{matrix} & (4)\end{matrix}$

In addition, the output current has intensity Iout which is equal toaverage of the secondary side current I2. The intensity Iout of theoutput current is given by:

$\begin{matrix}\begin{matrix}{{Iout} = {{\left( {2{Ton}} \right)/T} \times I\; 2}} \\{= {N\;{1/N}\; 2 \times {{Iin}.}}}\end{matrix} & (5)\end{matrix}$

The formula (5) is identical to the formula (2). This shows that theoutput current supplied to the output terminal 67 is fixed even if theduty ratio is varied on condition that the duty ratio is smaller than50%. Accordingly, it is possible to solve problems which occurs when theswitches S1 and S2 are in the on state at the same time. When theswitches S1 and S2 are in the on state at the same time, the primarywinding short circuits and the electric charges charged in the condenserC1 are suddenly discharged therefrom. The sudden discharge of thecondenser C1 is likely to bring any troubles to the transformer TR1and/or the condenser C1.

The submarine power feeding branching devices 53 a and 53 b whichcompanion to each other substantially equally share feeding power fed tothe branch submarine cable 54 a (54 b, 54 c, or 54 d) in the submarinepower feeding system of FIG. 1. Thus, the submarine power feedingbranching device 53 a must be adjusted and stabilized in the submarinepower feeding system of FIG. 1 so that the output current thereof hasthe same intensity as that of the output current from the submarinepower feeding branching device 53 b which companions thereto.

The description will be made about the adjustment for the submarinepower feeding branching device 53 a and stability thereof in thefollowing.

At first, the description is directed to the adjustment of the outputcurrent for the submarine power feeding branching device 53 a.

FIG. 9 is a timing chart of the switches S1 and S2. In FIG. 9, theswitches S1 and S2 are different from each other in duty ratio. FIG. 9also shows a waveform of the secondary side current in the secondarywinding of the transformer TR1.

Here, the duty ratio Rd1 of the switch S1 is represented by Ton1/T whilethe duty ratio Rd2 of the switch S2 is represented by Ton2/T. Inaddition, the period T is given by:T=Ton1+Ton2  (6).

The formula (3) can be rewritten as follow.Rd1+Rd2=1  (7)

The secondary current has intensity Iout1 during a period of Ton1 andintensity Iout2 during a period Ton2. Therefore, the secondary currenthas peak to peak intensity Ioutp-p given by:Ioutp−p=Iout1+Iout2  (8).

In addition, by the use of the turns ratio N2/N1, the peak to peakintensity Ioutp-p given by:Ioutp−p=N2/N1×Iin  (9).

Furthermore, because a direct current is not transferred from theprimary winding to the secondary winding, the following formula isvalid.Ton1×Iout1=Ton2×Iout2  (10)

The output current Iout, which is obtained by rectified, is representedby:Iout=(Ton1×Iout1+Ton2×Iout2)/T  (11).

By arranging the formulas (6) to (11), the following formula isobtained.Iout=4Rd1(1/Rd1)×N2/N1×Iin  (12).

From the formula (12), a relation between the duty ratio Rd1 of theswitch S1 and an output current-input current ratio Iout/Iin is derived.FIG. 10 shows a graph representing the derived relation between Rd1 andIout/Iin.

As understood from FIG. 10, when the duty ratio Rd1 is equal to 50%, thesecondary side current Iout has a maximum value of N2/N1×Iin. When theduty ratio Rd1 is equal to 0% or 100%, the secondary side current Iouthas a minimum value of zero. Thus, the secondary side current Ioutvaries according to the duty ratio Rd1 (and Rd2). In other words, thesecondary side current Iout can be controlled by the changing the dutyratios Rd1 and Rd2.

Next, the description is directed to the stability of the pair of thesubmarine power feeding branching devices 53 a and 53 b which areadjusted to match the output currents produced by the pair.

FIG. 11 is a graph representing a measured output voltage-currentcharacteristics of a current-current converter which can be used for theconstant current-constant current transformer T1. The outputvoltage-current characteristics has been measured by a measurementcircuit as shown in FIG. 12. As shown in FIG. 12, the measurementcircuit comprises a current generator connected to an input side of thecurrent-current converter. A variable resister is connected to an outputside of the current-current converter. The measurement has been made asresistance of the variable resister has been varied.

Returning to FIG. 11, the output current decreases from 609.8 [mA] to602.5 [mA] as the output voltage increases from 0.3 [V] to 41.1 [V].That is, the output voltage variation of 40.8 [V] (ΔV=41.1−0.3) is inconjunction with the output current variation of 7.3 [mA](ΔI=609.8−602.5). This is because a transformer for the current-currentconverter has output impedance which decreases the output voltage withthe increment of the output current. Here, a ratio of the output voltagevariation ΔV to the output current variation ΔI is referred to as aninclined resistance Rout (=ΔV/ΔI=5.6 kΩ).

The pair of the submarine power feeding branching devices 53 a and 53 bconnected to each other through the branch submarine cable 54 a (54 b,54 c, or 54 d) have such inclined resistance Rout each. Accordingly, thepair of the submarine power feeding branching devices 53 a and 53 b arestabilized by the inclined resistance Rout when they are adjusted tomatch their output currents to each other.

FIG. 13 shows output voltage-current characteristics of the submarinepower feeding branching devices 53 a and 53 b connected to each otherthrough the branch submarine cable 54 a together with their operatingvoltages and currents. For brevity's sake, load resistance R representsthe total of conductor resistance of the branch submarine cable 54 a andelectric resistance of the submarine repeater(s) interposed in thebranch submarine cable 54 a.

In FIG. 13, two graphs of the output voltage-current characteristics ofthe submarine power feeding branching devices 53 a and 53 b are labeled“CH-1” and “CH-2” respectively. Each of the submarine power feedingbranching devices 53 a and 53 b is restricted within a predeterminedoutput voltage. The submarine power feeding branching devices 53 a and53 b always have a common output current Iout because they are connectedin series.

As illustrated in FIG. 13, if the output current Iout is equal to acertain value 10, the submarine power feeding branching devices 53 a and53 b must produce the output voltages Vout1 and Vout2, respectively.Here, assuming that the submarine power feeding branching devices 53 aand 53 b forms a combined device, an output voltage Vout of the combineddevice is given by:Vout=Vout1+Vout2.

Accordingly, output voltage-current characteristics of the combineddevice can be obtained by the use of the output current Iout as aparameter. A graph of the output voltage-current characteristics of thecombined device are depicted in FIG. 13 and labeled “COMBINEDCHARACTERISTICS”.

In Consideration of the load resistance R, a graph of loadcharacteristics can be depicted in FIG. 13. The load characteristics aregiven by:Vout=R×Iout.

The combination device has an operation point which corresponds to anintersection between the graph of the output voltage-currentcharacteristics of the combined device and the graph of the loadcharacteristics. At the operation point, the output voltage has a valueof Voutop while the output current has a value of Ioutop. Operationvoltages Voutop1 and Voutop2 of the submarine power feeding branchingdevices 53 a and 53 b are obtained by the use of the output currentIoutop. Because the operation point is the intersection of the twographs, it is stable.

The inclined resistance described previously makes possible to find thestable operation point for the combined device. If each of the submarinepower feeding branching device of the combined device is an idealcurrent generator, the inclined resistance is infinite and the combineddevice can not share feeding power. That is, one of ideal currentgenerators is charged with the feeding power while the other has theoutput voltage of 0 [V] in such a case.

As mentioned above, by the use of the submarine power feeding branchingdevice 53 a having the structure of FIG. 6 and the power feedingbranching device 53 b having the same structure as shown in FIG. 6, thesubmarine power feeding system as illustrated in FIG. 5 can beconstructed.

In the submarine power feeding system of FIG. 5, the constant currentfeeding devices 51 a, 51 b and 51 c provided on land feed first constantcurrents to the submarine power feeding branching devices 53 a and 53 bthrough the trunk submarine cables 52 a, 52 b and 52 c. The submarinepower feeding branching devices 53 a and 53 b uses the first constantcurrents from the constant current feeding devices 51 a, 51 b and 51 cas power sources to feed second constant currents for the submarinerepeaters 55 through the branch submarine cables 54 a, 54 b, 54 c and 54d. The submarine power feeding branching devices 53 a and 53 b whichmake a pair produce the second constant currents flowing in oppositedirection and having identical intensity. The pair of the submarinepower feeding branching devices 53 a and 53 b about equally sharefeeding power between them. A plurality of the submarine power feedingbranching devices 53 a and/or 53 b can be provided along each trunksubmarine cables 52 a, 52 b or 52 c. Therefore, the trunk submarinecables 52 a, 52 b and 52 c and the branch submarine cables 54 a to 54 dcan be widely spread in mesh or lattice pattern. In consequence, thesubmarine repeaters (and the submarine observation devices) can bewidely arranged in second dimensional arrangement (or matrix).

As understood from FIG. 6, each of the submarine power feeding branchingdevices 53 a and 53 b can efficiently produce the second constantcurrent because it includes no element which wastes electric power.

Furthermore, each submarine power feeding branching device 53 a or 53 ballows electrical potential difference between the trunk submarine cableand the branch submarine cable which are connected thereto. This isbecause the submarine power feeding branching device is isolated betweenthe input side and the output side thereof. Therefore, the submarinepower feeding branching device make possible to construct various powerfeeding system flexibly.

In addition, the submarine power feeding branching device can producethe second constant current having equal intensity with the firstconstant current supplied thereto. Accordingly, the submarine powerfeeding branching device makes possible to construct the power feedingsystem that the first constant current on each trunk submarine cable isequivalent to the second constant current on each branch submarinecable. In such a system, the submarine repeater can be interposed ineither the trunk submarine cable or the branch submarine cable.

Referring to FIG. 14, the description will be made about a submarinepower feeding branching device according to a second embodiment of thisinvention. The submarine power feeding branching device 530 is similarto that (53 a) of FIG. 6 but has an additional resistor 612.

The additional resistor 612 is connected between the second outputterminal 67 and the ground terminal 68. The additional resistor 612reduces an inclined resistance R in comparison with that of thesubmarine power feeding branching device 53 a of FIG. 6. Hereby, acombined device comprising the submarine power feeding branching device530 and a similar device has a wide range of variable output current.

Referring to FIG. 15, the description will be made about a submarinepower feeding branching device according to a third embodiment of thisinvention. The submarine power feeding branching device 531 comprisestwo submarine power feeding branching devices 53 a of FIG. 6.

As shown in FIG. 15, the first output terminal 66 of the submarine powerfeeding branching device 53 a-1 is connected to the input terminal 65 ofthe submarine power feeding branching device 53 a-2. Furthermore, thefirst ground terminal 68 of the submarine power feeding branching device53 a-1 is connected to the second output terminal 67 of the submarinepower feeding branching device 53 a-2.

For normal operation, the submarine power feeding branching devices 53a-1 and 53 a-2 must produce identical output currents. This can be madeby controlling the duty ratio of the switches S1 and S2 in each constantcurrent-constant current converter 61. That is, in each of the submarinepower feeding branching devices 53 a-1 and 53 a-2, the output current isadjusted by controlling the duty ratio of the switches S1 and S2. Bothof the submarine power feeding branching devices 53 a-1 and 53 a-2 arestabilized by the effect of the inclined resistors of them. Thus, theoutput currents of the submarine power feeding branching devices 53 a-1and 53 a-2 matches with each other.

The submarine power feeding branching devices 531 can feed larger powerfor the branch submarine cable 54 a because it can produce higher outputvoltage in comparison with that (53 a) of FIG. 6.

Three or more submarine power feeding branching devices may be connectedin serial to produce further larger output voltage.

Referring to FIG. 16, the description will be made about a submarinepower feeding branching device according to a fourth embodiment of thisinvention. The submarine power feeding branching device 532 is similarto that (53 a) of FIG. 6 but has a transformer T2 instead of thetransformer T1. The transformer T2 differs from the transformer T1 inthat it has taps 613 and 614 provided along the primary winding. Thetaps 613 and 614 are equidistant from the midpoint of the primarywinding.

The submarine power feeding branching device 532 further comprises thirdto sixth switches S3-S6. The third switch S3 is connected between thefirst switch S1 and one end of the primary winding. The forth switch S4is connected between the first switch S1 and the tap 613 nearer the endof the primary winding that is connected to the third switch S3. Thefifth switch S5 is connected between the second switch S2 and the tap614 nearer the other end of the primary winding. The sixth switch S4 isconnected between the second switch S2 and the other end of the primarywinding.

The switch controller 62 performs a different operation to control notonly the switches S1 and S2 but also the switches S3-S6. The switchcontroller 62 receives a control signal transmitted from the landstation through the communication device 63 and controls the switchesS1-S6 according to the control signal.

While the switch controller 62 turns switches S3 and S6 on and turnsswitches S4 and S5 off, the submarine power feeding branching device 532can operate in the same manner as the submarine power feeding branchingdevice 53 a of FIG. 6. On the other hand, while the switch controller 62turns switches S3 and S6 off and turns switches S4 and S5 on, activelength of the primary winding is shorter than that of FIG. 6. That is,the turns ration N2/N1 is larger than that of FIG. 6 in this case. Thus,the submarine power feeding branching device 532 can have a wide rangeof variable output current in comparison with the submarine powerfeeding branching device 532.

While this invention has thus far been described in conjunction with thefew embodiments thereof, it will readily be possible for those skilledin the art to put this invention into practice in various other manners.For example, the submarine repeater may be provided along the trunksubmarine cable. Furthermore, a device such as the submarine powerfeeding branching device of FIG. 15 may be made by using the submarinepower feeding branching device of FIG. 14 or 16. In addition, thesubmarine power feeding branching device of FIG. 16 may comprise aresistor 612 as shown in FIG. 14.

1. A power feeding system including a plurality of trunk cablesconnected to feeding devices, a plurality of branch cables each of whichis provided between adjacent two of said trunk cables, and a pluralityof power feeding branching devices for connecting said branch cableswith said trunk cables, wherein each of said power feeding branchingdevices comprises: a constant current-constant current converter havingan input terminal, a first output terminal and a second output terminalwhich is electrically isolated from both said input terminal and saidfirst output terminal; and a controller connected to said constantcurrent-constant current converter for making said constantcurrent-constant current converter utilize a first constant currentsupplied to said input terminal to produce a second constant current anda restored first constant current which are to be supplied to saidsecond output terminal and said first output terminal, respectively. 2.A power feeding system as claimed in claim 1, wherein said power feedingbranching devices are classified into two types, one of the typesleading the second constant current from said constant current-constantcurrent converter to said second output terminal, the other of the typesleading the second constant current from said second output terminal toconstant current-constant current converter, and wherein: each of saidbranch cables is connected between two second output terminals of two ofsaid power feeding branching devices different from each other in type.3. A power feeding system as claimed in claim 2, wherein said powerfeeding branching devices connect said branch cables with said trunkcables to form a lattice pattern.